Thin-film, flexible photovoltaic module

ABSTRACT

A new, large-area, thin-film, flexible photovoltaic structure is disclosed, as well as a general fabrication procedure, including a preferably roll-to-roll-type, process-chamber-segregated, “continuous-motion”, method for producing such a structure. A special multi-material vapor-deposition environment is disclosed to implement an important co-evaporation, layer-deposition procedure performed in and as part of the fabrication procedure. A structural system adapted to create a vapor environment generally like that just referred to is disclosed, as is an organization of method steps involved in the generation of such a vapor environment. Also, a unique, vapor-creating, materials-distributing system, which includes specially designed heated crucibles with carefully arranged, spatially distributed, localized and generally point-like, heated-nozzle sources of different metallic vapors, and a special multi-fingered, comb-like, vapor-delivering manifold structure is shown.

This invention was made with Government support under Agreement No.MDA972-95-3-0036 awarded by DARPA. The Government has certain rights inthe invention.

TECHNICAL FIELD, BACKGROUND ART AND TECHNICAL ISSUES

1. Summary of the Invention

2. Field of the Invention

The system disclosed herein relates generally to the field ofphotovoltaics, and more specifically to fabrication of unique,thin-film, high-output, flexible chalcopyrite copper indium diselenide(CIGS) photovoltaic modules.

3. Background of the Invention

Looking briefly at the background for the field of the presentinvention, the field of photovoltaics generally relates to thedevelopment of multi-layer materials that convert sunlight directly intoDC electrical power. In the United States, photovoltaic (PV) devices arepopularly known as solar cells—which are typically configured as acooperating sandwich of p- and n-type semiconductors, wherein the n-typesemiconductor material (on one “side” of the sandwich) exhibits anexcess of electrons, and the p-type semiconductor material (on the other“side” of the sandwich) exhibits an excess of holes. Such a structure,when appropriately located electrical contacts are included, forms aworking PV cell. Sunlight incident on PV cells is absorbed in the p-typesemiconductor creating electron/hole pairs. By way of a natural internalelectric field created by sandwiching p- and n-type semiconductors,electrons created in the p-type material flow to the n-type materialwhere they are collected, resulting in a DC current flow between theopposite sides of the structure when the same is employed within anappropriate, closed electrical circuit. As a standalone device,conventional solar cells do not have a sufficient voltage required topower most applications. As a result, conventional solar cells arearranged into PV modules by connecting the front of one cell to the backof another, thereby adding the voltages of the individual cellstogether. Typically a large number of cells, on the order to 36 to 50are required to be connected in series to achieve a nominal usablevoltage of 12 to 18 V.

Commercial use and interest in thin film photovoltaics (PV) has expandeddramatically in the last five years, however commercial wide-scale usefor bulk power generation remains limited primarily by two factors: (1)performance and (2) cost. Over the past few years dramatic improvementsin PV module performance have been achieved in both crystalline siliconand thin film photovoltaics. Laboratory scale efficiency of crystallinesilicon is approaching 20%. Modules ranging from 10 to 14% arecommercially available from several vendors. Similarly, laboratory scaleefficiencies of well above 10% have been achieved with thin film PVdevices of copper indium diselenide, cadmium telluride, and amorphoussilicon, most notably the record efficiency for CIGS is now approaching19%. Additionally several companies have achieved thin film large areamodule efficiencies ranging from 8 to 12%. As a result of these recentadvances, performance no longer seems to be the key limiting factor,leaving cost as the primary factor preventing wide-scale commercial useof PV modules for electricity generation.

Thin film based photovoltaics, namely amorphous silicon, cadmiumtelluride, and copper indium diselenide, offer improved cost byemploying deposition techniques widely used in the thin film industryfor protective, decorative, and functional coatings. Common examples oflow cost commercial thin film products include water permeable coatingson polymer-based food packaging, decorative coatings on architecturalglass, low emissivity thermal control coatings on residential andcommercial glass, and scratch and anti-reflective coatings on eyewear.

Copper indium gallium diselenide (CIGS) has demonstrated the greatestpotential for high performance, low cost thin film PV products. Of thethin-film PV, CIGS has achieved the highest laboratory efficiency (18.8%by NREL), is stable, has low toxicity, and is truly thin film (requiringless than two microns layer thickness). The above factors all lead to ahigh potential for CIGS to be economically manufactured on a largescale, thereby penetrating bulk power generation markets.

Thus far, however, and remarking here briefly on a de facto prior artshortcoming that relates to attractive and encouraging use of CIGS/CISmaterials, the majority of CIGS/CIS research and development has beenconcentrated to date on the deposition of thin-film layers of thesematerials on rigid glass substrates. Key attributes of glass are thecost of the base raw material, good coefficient of thermal expansionmatch with the CIGS device layers, and vacuum deposition system designsexist. Such rigid substrates are not ideal in many circumstances, and donot lend themselves readily to large-volume, high-yield, commerciallymanufacturing of multi-layer functional thin film materials such asphotovoltaics.

Major contributing factors of rigid substrate processing on cost are:

1. Large scale processing of glass requires substantial floor space for2-dimensional (x, y) processing equipment and material storage,

2. Large sections of plate glass require specialized heavy duty handlingequipment,

3. Continuous, automated in-line processing of a multi-layer structure(4-layers for CIGS), requires several additional chambers to isolate thelayer-specific processing environment, a feature not encountered in mostglass processing industries,

4. Heating and cooling rates between processing zones is slow and mustbe carefully controlled to avoid fracturing the glass,

5. Even with careful control of process heat and cooling times, glassfracturing often occurs and has a tremendous impact on yield,

6. Volume and weight of the product impact shipping cost to the usesite, substantially increasing cost,

7. Installation, especially rooftop, requires additional labor (i.e.,two-persons) to place the panels.

Roll-to-roll processing of thin flexible substrates addresses all theabove issues. Key attributes of roll-to-roll processing of thin flexiblesubstrates include:

1. Compact, less expensive vacuum systems,

2. Complete process isolation based on batch processing of long rolls,

3. Extreme tolerance to rapid heating and cooling as well as largethermal gradients,

4. Robust structure is unlikely to fracture or fail during processing,

In choosing flexible processing, a few issues are encountered that areaddressed in the present system: (1) Molybdenum-deposition(Mo-deposition) approaches used for glass are unacceptable for flexiblesubstrates, and (2) CIGS processing using traditional industrialapproaches are not logical for continuous roll-to-roll processing.

SUMMARY OF THE INVENTION

The overall efforts which surround the developments that arespecifically addressed in this document have introduced, to the field ofphotovoltaics, a significant collection of innovations that apply toimproved low-cost, large-scale manufacturing of thin film CIGSphotovoltaic, more specifically flexible, thin-film modules. Generallyspeaking, several key areas of these innovations include: (1), a newlarge-area, thin-film, flexible photovoltaic structure; (2), a generalfabrication procedure, including a preferably roll-to-roll-type,process-chamber-segregated, “continuous-motion”, method for producingsuch a structure; (3), a special multi-material vapor-depositionenvironment which is created to implement an important co-evaporation,layer, deposition procedure performed in and as at least part of themethod just mentioned; (4), a structural system uniquely focused oncreating a vapor environment generally like that just referred to; (5),an organization of method steps involved in the generation of such avapor environment; (6), a unique, vapor-creating, materials-distributingsystem, which includes specially designed heated crucibles withcarefully arranged, spatially distributed, localized and generallypoint-like, heated nozzle sources of different metallic vapors, and aspecial multi-fingered comb-like, vapor-delivering manifold structure;and (7), other matters.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a simplified schematic elevation generally illustratingprocess steps and stages employed for creating a PV module.

FIG. 2 is a fragmentary, plan view, taken generally in the direction ofarrow 2—2 in FIG. 1, showing a piece of a roll of long and thin stripmaterial employed in the disclosed system, with this piece beingillustrated as containing portions of three boundary-defined, PV cellsstill resident in the strip material.

FIG. 3 is a further-enlarged, fragmentary, simplified cross section,taken generally along the line 3—3 in FIG. 1, illustrating one form of amultiple-layer construction that makes up a photovoltaic device.

FIG. 4 illustrates an alternative photovoltaic device layerconstruction.

FIG. 5 is a simplified, schematic side elevation illustratingroll-to-roll, Mo-deposition, strip processing that takes place withinone of the several, separated chambers that are employed in theproduction of a PV device constructed in accordance with the disclosedsystem.

FIG. 6, is a simplified, schematic side elevation illustrating a chamberwherein one format of CIGS or CIS roll-to-roll strip processing occursaccording to the disclosed embodiment, with this one format involvingmovement of strip material past three vapor-plume-creating stations.

FIG. 7 is a schematic, perspective and partially-fragmented view of adeposition zone R which exists in the chamber pictured in FIG. 6.

FIG. 8 is a schematic section view taken generally along the line 8—8 inFIG. 7.

FIG. 9 is a fragmentary plan view generally along the line 9—9 in FIG.6.

FIG. 10 is a fragmentary schematic perspective view of avapor-deposition zone R wherein certain vapor-creating andvapor-presence phenomena exist and take place within the chamberpictured in FIGS. 6, 7 and 8.

FIG. 11 illustrates two or three roll-to-roll chamber-processing steps.Specifically, FIG. 11 can be viewed as illustrating (a) a step ofapplying a layer of CdS, (b) a step, when employed, of applying a layerof i-ZnO, and (c) a step of applying a conductive-oxide layer, such as aZnO:Al layer. This single-view, “multiple-purpose” drawing figure isemployed in the interest of simplifying the overall collection ofdrawings.

FIG. 12 presents a schematic side elevation view of an alternativechamber wherein another format of CIGS or CIS roll-to-roll stripprocessing occurs according to another embodiment of the presentinvention, with this other format involving movement of strip expansespast five vapor-creating stations.

FIG. 13 is a schematic, perspective view of the chamber of FIG. 12.

FIG. 14 is a schematic sectional view taken generally along the line14—14 in FIG. 13.

FIG. 15 schematically illustrates the basic component elements(including the selenium delivery elements) of a nozzle-basedvapor-delivery apparatus according to the disclosed system, with thisapparatus being shown isolated and removed from the chamber of FIG. 6.

FIG. 16 is a fragmentary plan viewed of an upper left portion of thechamber of FIG. 9.

FIG. 17 is a fragmentary plan view illustrating portions of one of thenozzle-bearing crucibles pictured in FIGS. 6-16 employed for thedelivery of copper, gallium and indium vapors.

FIG. 18 is an end elevation taken from the bottom side of FIG. 17, alsowith certain regions broken away to show inside details.

FIG. 19 is an enlarged, fragmentary cross section taken generally alongthe line 19—19 in FIG. 17, showing details of a vapor-plume-generatingnozzle in the crucible pictured in FIGS. 9 and 16-18.

FIG. 20 is an enlarged, fragmentary cross-section taken generally alongthe line 20—20 in FIG. 16, showing an outlet port or nozzle in a spargertube, or finger, which forms part of a comb-like manifold structure thatfunctions to deliver selenium vapor in the disclosed embodiment.

FIG. 21 is a fragmentary cross section taken generally along the line21—21 in FIG. 16.

FIG. 22 is a graph picturing, generally, the ratio of copper togallium-plus-indium at different locations along the length ofdeposition zone R. The left side of FIG. 22 relates to the entry end ofzone R.

FIG. 23 is a graph illustrating, generally, the ratio of gallium togallium-plus-indium at different points along the length of zone R, withthis graph picturing, at its left-hand side, conditions at the entry endof zone R.

FIGS. 24, 25 and 26 present graphs that relate the effusion rates (ingrams per hour) as a function of molten-material temperature for copper,gallium and indium, respectively.

FIGS. 27, 28 and 29 in a simplified schematic manner, illustrate severalalternative ways in which deposition fog that is produced by employingthe vapor-plume apparatus of the present system can be “delivered” (in adirectional sense) for creating a thin-film layer on a surface in astrip of reception material.

FIG. 30 is a fragmentary plan view illustrating portions of thevapor-delivery system in the chamber of FIGS. 12-14.

FIG. 31 pictorially illustrates flow of the vapor through an orifice.

FIG. 32 depicts the molecular beam profile from an effusion source.

FIG. 33 illustrates the parameters of equation 14.

FIG. 34 illustrates transverse thickness profiles for two and threeorifice systems.

FIG. 35 illustrates the instantaneous flux of copper gallium and indiumas a strip passes through the deposition zone.

FIG. 36 illustrates the instantaneous ratio of Cu/(ln+Ga) as the strippasses through the deposition zone.

FIG. 37 illustrates the instantaneous ratio of Ga/(ln+Ga) as the strippasses through the deposition zone.

FIG. 38 illustrates the cumulative ratio of Cu/(In+Ga) as the strippasses through the deposition zone.

FIG. 39 illustrates the cumulative ratio of Ga/(In+Ga) as the strippasses through the deposition zone.

FIG. 40 illustrates the instantaneous flux of constituents as the strippasses through the deposition zone.

DETAILED DESCRIPTION

FIG. 1 illustrates the steps involved preferably in making a new kind ofPV module in a roll-to-roll, continuous-motion in accordance with thepresent invention. Those skilled in the art should understand that,while roll-to-roll, continuous motion processing is employed in thedescribed embodiment, non-roll-to-roll procedures could be usedeffectively in certain applications.

FIG. 1 shows two rolls 10, 12, at the left and right, respectively,which symbolize the several roll-to-roll, continuous-motion processingstages employed in the manufacturing of this new kind of PV module. Roll10 represents a pay-out roll, and roll 12, a take-up roll. It should beunderstood that rolls 10, 12 are representative of the different pay-outand take-up rolls that are employed in different isolated processingchambers. Thus, there are typically multiple pay-out and take-up rollsused during the overall process.

A stretched-out, flat portion of an elongate strip of thin, flexible,substrate material 14 is shown extending between rolls 10, 12. Thissubstrate strip has different amounts of applied (deposited) PV-celllayer structure at different positions between the rolls. The strip hasopposite end winds that are distributed as turns on pay-out roll 10 andtake-up roll 12. The direction of travel of the strip material duringprocessing is indicated generally by arrow 16. Curved arrows 18, 20indicate, symbolically, the related, associated directions of rotationof rolls 10, 12 about axes 10 a, 12 a, respectively, which are generallynormal to the plane of FIG. 1.

Reference herein to the substrate strip material 14 should be understoodto be reference to a strip of material whose overall structuralcharacter changes as the material travels, in accordance with processingsteps, between rolls 10 and 12. Through the processing steps, layers ofvarious components that go into the fabrication of the type of PV-moduleare added.

Nine separate individual processing chambers 22, 24, 26 23, 25, 27, 28,30, 29 are illustrated as rectangular blocks in FIG. 1. The variouslayers of materials that are used to form a PV module according to thisinvention are applied or modified in these chambers. The relative sizesof these blocks as pictured in FIG. 1 are not important. It should benoted that the steps represented by some of the processing chambers areoptional in some applications. For instance, the i-ZnO layer created inchamber 28 may be omitted.

Processing begins with a bare starter roll, or strip, of elongatethin-film, flexible substrate material, preferably polyimide (PI), maybe supplied from pay-out roll 10. This uncoated material might typicallyhave a width of about 33-cm, a thickness of about 0.005-cm, and a lengthof up to about 300-meters. The width, thickness and length dimensionsare, of course, matters of choice, depending on the ultimate intendedapplication for finished PV modules. One PI material suitable for use inthe disclosed system, is Upilex S—a material currently availablecommercially from KISCO in Santa Clara, Calif.

PI is a suitable supporting substrate because it (1) can be made verythin, and thus can offer good flexibility, and (2) can toleraterelatively high-temperature environments without sustaining damage. PImaterial also is quite widely commercially available, and is relativelyinexpensive. Of course other materials having similar physicalproperties, such as any high-temperature polymer, or a thin metal suchas stainless steel, titanium, covar, invar, tantalum, brass and niobiumetc, can also be used with appropriate process modifications.

A fragment of such a starter strip of PI is illustrated generally onedge at 32, immediately above roll 10. PI fragment 32 advances to theright in FIG. 1 through the several processing environments representedin this figure, and is referred to throughout the discussion of thisfigure with the same reference numeral 32.

A stress-compliant metal interlayer, preferably Ni-V, chosen to haveintermediate thermal expansion characteristics between the PI and asubsequently-applied Mo layer(s) can be optionally utilized as the firstlayer deposited onto the PI. This step is not illustrated in FIG. 1, butcan either be accomplished in a chamber similar in construction tochamber 22 or within a separate processing zone in chamber 22.

Within chamber 22, and in a manner that will be more fully discussedshortly, two layers of Mo, each containing entrained oxygen, and eachpossessing a certain level of desirable, internal compressive stress areformed on the opposite sides, or faces, of PI strip 32. These two layersare shown on the opposite faces (top and bottom in FIG. 1) of fragment32 at 34, 36 above chamber 22. Layer 34 forms a back contact layer forthe PV module of the present invention. In the case of a stainless steelsubstrate strip, the Mo back contact layer would normally be replacedwith a Cr/Mo bilayer.

Material emerging from Mo-processing in chamber 22 is ready forintroduction into chamber 24, wherein it is exposed to a unique vapordeposition environment created in accordance with the current inventionto create an absorber layer.

In chamber 24, a multi-element crystalline absorber layer 38 formed onMo layer 34 by a unique multi-source co-evaporation technique.Preferably, layer 38 is p-type semiconductor in the form ofcopper-indium-gallium-diselenide (CIGS), or its readily acceptablecounterpart, copper-indium-diselenide (CIS). For purposes of simplicity,the following discussion will generally utilize CIS or CIGS to refer toCIS with or without metal alloys such as gallium, aluminum, boron, orsulfur. These different compositions, among others, can be usedessentially interchangeably as an absorber layer in various embodimentsof the invention depending on the particular properties desired in thefinal product.

It may be useful at this point to preview certain matters regardinglayer 38. First, as a direct consequence of the particular co-depositionevents that take place in the unique fog environment that exists inchamber 24, layer 38 has a consistent multi-element-compositionalmake-up and uniform thickness throughout. This applies bothlongitudinally over the length of the substrate and side-to-side acrossthe substrate. Next, adhesion of the CIGS layer to the receiving surfaceof the pre Mo layer is excellent. The adhesion occurs with (a) noappreciable fabrication-caused damage to the Mo layer, and (b) formationof a proper-constituent content, single-crystalline structure in theCIGS layer. Lastly, these benefits are achieved using a relativelysimple, single-deposition-chamber operation.

The configuration of the stack of layers after emergence of thesubstrate from chamber 24 is shown above chamber 24. Note that in thisfragmentary edge view, only the most recently added layer (38) isdesignated with a layer reference numeral, along with PI designator 32.This labeling approach is used throughout the remainder of thedescription provided herein for FIG. 1.

In chamber 26, a window or buffer layer in the form of cadmium-sulfide(CdS) is applied as a layer 40 extending over the CIGS or CIS layer thatwas formed in chamber 24. The CdS layer is preferably applied in anon-wet manner by radio-frequency (RF) sputtering. This results in anoverall multiple-layer structure such pictured generally above chamber26.

After deposition of the Mo, CIGS, and CdS, the strip proceeds through asequence of operations, 23, 25, 27 designed to first divide, thensubsequently, serially connect adjacent ‘divided’ areas. The firstoperation is to scribe through all deposited layers exposing bare,uncoated PI. This first scribe functionally divides the elongate stripof deposited layers into plural individual segments and thereby isolateseach segment electrically. These segments are held together by the PI,which remains intact. The scribing technique used is a matter of choice,with the preferred method herein accomplished using a high power densitylaser.

Directly adjacent to the first scribe operation, a second selectivescribe is conducted removing the CdS and CIGS layers but leaving the Mointact in the as-deposited conditions. This selective scribe forms avia, or channel, that will be later filled in with a conductive oxide.

To prevent the conductive oxide in the top contact layer from ‘fillingin’ the first scribe, Mo/CIGS/CdS, and in effect reconnecting adjacentdivided Mo regions, the scribe must be filled in with an insulator.Preferably, this is accomplished with a UV curable ink deposited inoperation 27 with a commercially adapted ink jet dispense head that iscoincident with the high power density laser.

If the optional, electrically insulating, intrinsic-zinc-oxide i-ZnOlayer is employed, this is prepared in processing chamber 28 to create alayer arrangement such as that pictured above chamber 28. In this layerarrangement, the i-ZnO layer is shown at 42, overlying the CdS layer.

A top contact layer in the form of a transparent, conductive-oxideoverlayer 44, such as ITO or ZnO:Al layer, is formed in processingchamber 30, either directly upon CdS layer 40 in no i-ZnO layer is used,or directly on i-ZnO layer 42 where it is present. The resultingcomposite layer structure is indicated generally above chamber 30 inFIG. 1.

Where an insulating i-ZnO layer, such as layer 42, is created, theresulting overall layer structure includes what is referred to laterherein as a sandwich substructure, indicated generally by arrows 46 inFIG. 1. Substructure 46 includes the i-ZnO layer sandwiched between theCdS layer and the ZnO:Al layer. Thus, where such a sandwich substructureis employed, a contiguous protective intermediary layer (i-ZnO) isprovided interposed between the CIGS/CIS layer and the top contact layer44.

FIG. 3 illustrates a PV-cell structure 47, such as produced from theprocess outlined in FIG. 1. It should be noted that the layerthicknesses are not drawn to scale. The specific layer arrangement whichmakes up device 47 includes, a stress neutralizing back side coating, aPI substrate 32, a stress compliant-intermediate CTE Ni-alloy layer,oxygen-entraining and internally-compressed Mo layers 34, 36, CIGS orCIS layer 38, CdS layer 40, i-ZnO layer 42, and overlying, transparentconductive-oxide, ZnO:Al layer 44. This is a device wherein the optionto employ i-ZnO has been elected. The sandwich substructure 46 mentionedearlier, which includes this i-ZnO layer, is identified with a bracketwhich bears reference numeral 46 in FIG. 3.

FIG. 4 illustrates the upper-layer portion of another PV-cell structure49. Cell structure 49 differs from cell structure 47 by not containingan i-ZnO layer. Thus, in device 49, conductive-oxide layer 44 liesdirectly on and in contact with CdS layer 40. Those skilled in the artwill recognize, and be familiar with, the particular kinds ofapplications or situations wherein a CdS layer alone can be utilizedwithout the intermediary i-ZnO layer.

The product of the above-described process is a series of long narrowcells of active PV material monolithically interconnected along theiredges such that the top of a cell becomes electrically connected to thebottom of the next cell. The result is a chain of cells electricallyconnected in series to generated a desired cumulative voltage for agiven product application. The resulting plurality of edge-to-edgeseries interconnected, thin-film, flexible PV cells, are schematicallydepicted in FIG. 2 as 50, 51 and 52. These three cells are separated bylines that represent scribes 41, 45 and 47. Scribe 41 is overcoated withthe ink-jet deposited, UV curable ink. Each of these cells has “plan” or“footprint” dimensions of about 33-cm by about 0.4 to 1-cm, mostpreferably in the range 0.5 to 0.6-cm. Representative scribes 41, 45, 47are about 50 microns wide and extend the entire cell length of 33-cm.The ink-jet deposited insulator, 43, overlying scribe 41 has dimensionsof about 50 to 200-microns wide, with the optimum dimension of about 80to 125-microns, and extends the entire cell width of 33-cm. It should benoted that each of cells 50, 51, 52 in FIG. 2 includes the optionali-ZnO layer. The previously described layers in the module—PI substrate32, Mo back contact layers 34 and 36, CIS absorber layer 38, CdS bufferlayer 40, i-ZnO insulating layer 42, and ZnO:Al top contact layer 44—areillustrated as separated by the six cutaway lines in FIG. 2.

Monolithic integration creates a large building block, referred to as asubmodule used to create a final module with desired voltage and currentfor the expected application. Building block size 30×30-cm but is, ofcourse, a matter of choice depending on the starting strip material sizeand the anticipated end-use of the product. At appropriate points intime during the overall processing procedure which has just beengenerally outlined in the ‘monolithic interconnection’ discussion, thefunctional electrical circuitry which is required in the end-product PVmodule structure is prepared. This circuitry establishes the neededelectrical interconnects between adjacent submodules. The specificmonolithic interconnection patterning configuration employed, and thetechnique(s) for creating such a configuration, are matters of choice,and are well known to those versed in the art.

FIGS. 5-8 and 10-14 each pictures schematically one of the processingchambers described generally above. It should be noted that the chamberpictured in FIG. 11 is readable to illustrate two or three of theprocessing steps performed in the practice of this invention, dependingupon whether the optional i-ZnO layer is created. The enclosures formingthe processing chambers are conventional in construction.

The short open arrows employed in FIGS. 5, 6, 11 and 12 representvarious conventional hardware components that are involved in theintroduction of substances and control parameters employed within theenvironment of the respective associated chamber. For instance, thearrow for chamber 22 represents the various conduits, valves, nozzlesand controllers that are conventionally used to introduce the gas/vaporconstituents utilized in chamber 22. Pay-out/take-up, roll-to-rolltransport systems are also depicted schematically in these figures. Thedetails of these systems are not illustrated, inasmuch as such detailscan take any one of a large number of different forms that are wellknown to those skilled in the relevant arts. Thus, appropriate guiderollers, tensioners, stationary guides, and other devices that wouldmake up the construction, typically, of a suitable roll-to-rolltransport system, are omitted from the drawing figures, and are notdiscussed herein in any detail.

Also pictured within the chamber-representing blocks shown in FIGS. 5-7and 11, as well as in FIGS. 8, 13 and 14, are certain otherschematically-represented, process-implementing components that playdefined roles in the specific activities that take place within therespective associated chambers, as will be described below.

Unprocessed strip material which pays out from roll 58 is initiallyheated to a temperature within the range of about 100° to about 500° C.,and preferably to about 300° C. Heating drives unwanted moisture (water)out of the PI material in preparation for uniform back contactdeposition. Heating is performed in a heating arena 63 sized in relationto strip transport speed to assure proper pre-Mo-deposition drying ofthe PI strip material. Heat is supplied in the heating arena by a pairof elongate, resistive, heating filaments, such as heating elements madeavailable from the Omega Corporation, or from Watlow of Cleveland, Ohio.The elements are serpentine structures embedded in metal plates on theorder of about 15-cm to about 150-cm in length, and most preferablyabout 60-cm long. Thus, at a web speed of approximately 30-cm perminute, each point on the PI strip material remains within the heatingarena 63 for approximately two minutes. Commonly employed alternateprocesses such as direct current plasma treating may also be used toremove unwanted water from the polymeric strip material prior to backcontact deposition.

Driving out moisture contained in the “starter” PI material andcarefully monitoring and controlling the input of water vaporfacilitates precise control over the process of entraining oxygen.

Processing proceeds with the back contact sputter deposited on the driedweb in chamber 22. Two options exist, (1) first depositing the stresscompliant layer in a separate chamber virtually identical to the chamber22 or directly in line with the Mo deposition in chamber 22, or (2)depositing the Mo directly on the PI web. In either case, Mo isdeposited on both sides of the PI strip and contains the oxygenentrapment.

Whether processing the Ni-based stress compliant layer or the Mo withinchamber 22, a pay-out roll 58 of PI strip material, such as thatidentified earlier, feeds a downstream take-up roll 60, with a longreach or length 62 of this material extending between these two rolls.With a PI material having dimensions like those described above, theinitial diameter of roll 58 is typically about 25-cm. Preferably, thelong flat strip 62 which extends between rolls 58, 60 is maintained at atension in the range of about 0.1- to about 10-kg, and preferably withinthe somewhat smaller range of about 1.0- to about 5.0-kg. Mostparticularly, a strip tension in this chamber of approximately 2.0-kg issuitable. The linear transport speed of the material within chamber 22is generally between about 1-cm-per-minute and about 5 m-per-minute. Atransport speed of about 30-cm-per-minute is typical. Transport speed,of course, can be varied as a matter of technical choice.

The environment within chamber 22 during processing is typicallymaintained at a controlled vacuum, ultimately, of approximately between1.0 and 15 millitorr, but preferable 2-milli-Torr. Preferably, theinterior of chamber 22 is evacuated initially to a pressure on the orderof about 1.0×10⁻⁶ Torr, whereupon argon gas is introduced until thepressure in the chamber rises to approximately the 2-milli-Torr levelmentioned above. This pressure level is then maintained to within aboutplus or minus 0.5-milli-Torr, and preferably to within about plus orminus 0.1-milli-Torr.

After heat treating processing begins by transporting the driedpolyimide material through chamber 22. Ni-V alloy has proven to besuitable for the metal interlayer. However any stress compliant metal,with a coefficient of thermal expansion (CTE) intermediate to that ofthe flexible substrate and the back electrical contact could beemployed. Preferred materials are nickel based alloys flat have enoughalloying element to render the Ni non-magnetic but retain the Ni stresscompliance, intermediate CTE, low bulk resistivity, and doesn't reactwith the overlying Mo. Examples include nickel alloyed with 0 to 10weight percent vanadium, nickel alloyed with 0 to 15 weight percentmolybdenum, and Ni alloyed with 0 to 7 weight percent chromium.Additionally, alternative metals that have the characteristic ofintermediate CTE and low resistivity could be used. Examples includecopper and copper alloys including brass, niobium, chromium, tantalum,and titanium.

In each of stations 64, 66, the Ni alloy source employed for the stresscompliant layer sputter-deposition preferably takes the form of a 99.95%sintered Ni with 7 weight percent vanadium block, for example, which iscommercially available from Pure Tech Inc., Carmel, N.Y. For this Ni-7Vsource material the sputtering cathodes are suitably (andconventionally) operated at power levels ranging from 1 to 10 kW each,with 4.0-kW each being most preferable.

Processing further proceeds with the formation/deposition of the twopreviously discussed Mo layers on opposing faces of the PI web. The Molayers are formed by a Mo-deposition plasma generated inside the chamber22 (FIG. 5). In each of stations 64, 66, the Mo source employed for Mosputter-deposition preferably takes the form of a 99.95% pure vacuum arccast Mo block, for example, one which is commercially available fromClimax Specialty Metals of Cleveland, Ohio. For this Mo source materialthe sputtering cathodes are suitably (and conventionally) operated atpower levels ranging from 1 to 10 kW each, with 4.0-kW each being mostpreferable.

In each of sputtering stations 64, 66, the spacing between the stripmaterial and the Mo source material is arranged to be about 1- to about20-cm, and most preferably about 10-cm. These spacing considerationsplay an important role in ensuring that the local deposition regionsdefined between the substrate and the Mo source material are especiallysuited to promote the introduction of appropriate compressive stressinto the forming Mo layers. Within these regions, the pressure of argongas increases and decreases, respectively, as the distance between thesubstrate and the sources decreases and increases. The preferablespacing distances and the chamber pressures stated above assure acondition in chamber 22 which causes the activity of argon atoms topromote a slight, but desirable, level of internal compressive stress ineach of the two, forming Mo layers. It is believed that this compressivestress comes into being as a result of the fact that argon atoms“hammer” and effectively “ball-peen” the forming Mo layers. As describedin more detail below, the internal compressive stress tends tocounteract the tension imparted in the Mo layers that would otherwiseoccur as the strip is heated and the PI expands during the CIGSprocessing described later.

It is important to note that the use of conventional sputteringtechniques for depositing a Mo layer on a PI substrate can result in acomposite structure that is insufficiently ductile. That is, normalsputtering techniques will typically result in a Mo layer which isbrittle, and which is therefore a likely candidate for fracturing andfailure during subsequent processing steps.

We have found, as at least one component of the present invention, thata solution to this problem involves the addition of the mentionedentrained oxygen to the Mo layers. It is believed that the oxygenintroduced into chamber 22 according to the present invention occupiesthe interstitial site of the BCC unit cell. Oxygen present atinterstitial sites creates a higher level of internal compressivestress. Because the coefficient of thermal expansion of the PI is muchgreater that of the Mo, the high level of compressive stress isnecessary to ensure the Mo remains in compression during heating in theCIGS chamber. If the Mo is not subject to enough compression, thethermal expansion mismatch between the PI and Mo would cause the Mo totransition into tension in the CIGS chamber and create cracking withinthe Mo. The introduced oxygen contributes to the act of creatinginternal compressive stress in the Mo layers. As a consequence, the Molayers become more tolerant to bending and more resistant to cracking.

Thus, two principles are applied here to increase the compressive stresswithin the Mo: 1.) the combination of low sputtering pressure and shorttarget-to-substrate spacing creates reflected neutral argon peeningwhich imparts an intrinsic high state of compression in the Mo film; and2.) the addition of oxygen to create a higher level of internalcompressive stress in the Mo.

Under static vacuum conditions, i.e., while maintaining a closelycontrolled vacuum on the order of about 10 milli-Torr, within chamber22, an oxygen/argon environmental condition develops with (as mentionedearlier) an optimum molar ratio of argon atoms to oxygen molecule on theorder of between about 5-to-1 and about 50-to-1. Most preferably, thisratio is about 20-to-1. That is, for every 20-argon-atoms present withinthe chamber, approximately 1-oxygen-molecule exists; for every15-argon-atoms within the chamber, there is approximately 1-oxygen-atom.With the environmental parameters established as above stated withinchamber 22, the two, desired, oxygen-entraining Mo layers result with anappropriate level, in each layer, of compressive stress.

The resulting thickness of Mo layers 34, 36 can be monitored in anyappropriate conventional manner as, for example, in an indirect mannerby employing a standard Quartz Crystal Monitor (QCM) which can beconfigured to measure the rate of sputtering (rate of build-up) ofsputtered materials in terms of angstroms-per-second, in situ. Asuitable standard QCM is available from Liebold Inficom of Syracuse,N.Y. An end result thickness for this layer which has been found to beentirely satisfactory is about 0.5-micrometers, resulting in a layersheet resistance of about 0.75-ohms-per-square.

Regardless of the precise physical mechanism(s) by which the presencesof oxygen and argon cause these elements to assert themselves on, and inrelation to, the formation of Mo layers 34, 36, important end resultsare: (a) that each of these layers bonds strongly to its associated PIstrip face; and (b) that these layers are able to tolerate temperaturechanges that occur in subsequent processing without sufferingtemperature-induced cracking and fracturing. Additionally, layers 34,36, disposed as they are on the opposite faces of the PI strip material,mechanically “balance” one another to inhibit product curling, or“bending out of plane.” Such bending could be a problem and/or aninconvenience if only a single Mo layer were used. By way of example,the induced internal compression in a single Mo layer would besufficient to curl the substrate to the diameter of a pencil without thebalancing effect of the opposite layer.

Oxygen is preferably made available for entrainment into the Mo layersby controlled, variable-rate introduction of heated (foranti-condensation purposes) water vapor into chamber 22. Although thereare several techniques available to those skilled in the art ofsputtering, specifically, we have found that a vapor source mass flowcontroller as supplied by MKS Instruments of Boulder, Colo. is suitablemechanism for the introduction of the water vapor. Typical water flowrate is 0.1 to 10 sccm with a preferable range of 1.5 to 2.5 sccm. Bymaintaining in chamber 22 a molar ratio in the range of about 5:1 toabout 50:1 for argon-to-oxygen atoms, appropriate entrainment of oxygenis attained, although values outside this range may give adequateresults as well. Where, instead of using introduced water vapor, gaseousoxygen is employed according to a practice of the present invention,such oxygen is preferably delivered into chamber 22 from an externalsupply tank typically at approximately 10-cm³-per-minute. Argon gas froman external supply tank is fed into chamber 22 preferably at a rate ofabout 200-cm³-per-minute.

The heated and dried PI substrate is passed through first and second Mosputtering stations, shown generally at 64, 66, respectively. It iswithin these two stations that oxygen is introduced to the forming Molayers and argon acts with such added oxygen, to create a level ofinternal compressive stress within the Mo layers. The creation of thiscompressive stress is discussed in more detail above. In station 64,what can be thought of as the “back side” of the substrate (the upperside of stretch 62 in FIG. 5) is sputter-coated with Mo to create alayer like previously-mentioned layer 36. In station 66, the opposite(lower) side of the substrate is also so coated, here to create aPV-active Mo layer, like previously mentioned layer 34

When the entire strip of material has finished its processing transportwithin chamber 22, and has been collected on take-up roll 60, the latteris removed from chamber 22, and placed in chamber 24 (see now FIGS. 6-8,10 and 12-14) to become the operative pay-out roll in that chamber. Thestrip material feeds in the direction of arrow 16 from pay-out roll 60to a downstream take-up roll 68 in chamber 24. As the strip materialmoves through chamber 24, the absorber CIGS or CIS layer 38 is formed onMo layer 34. A transport-guide structure (not shown) is employed betweenrolls 60, 68 in chamber 24 to support and guide the strip. The short,open arrow which appears at the left side of the block representation ofchamber 24 in FIG. 6 symbolizes the hardware provided for the deliveryof appropriate constituent substances to chamber 24.

It is within chamber 24, and specifically within a deposition zone R,that the unique molten-liquid-to-vapor co-evaporation process mentionedabove for establishing a CIGS or a CIS layer is performed. FIGS. 6-8 and10 illustrate schematically a configuration for, and certainenvironmental conditions within, the inside of chamber 24. Chamber 24 isdesigned specifically for the creation (according to one way ofpracticing this stage of the present invention) of a CIGS (rather than aCIS) layer. Accordingly, pictured as small blocks and tiny circles (FIG.6 only) distributed along the bottom of chamber 24, are structures,designated 70, 72, 74, 76, 78, 79, 81, which function to generate vaporsof copper (70), gallium (72) indium (74) and selenium (76, 78, 79, 81)for deposition. Structures 70-81 form the bulk of thevapor-deposition-creating system 83 of the present invention. One of thefeatures that distinguishes this embodiment is that the vapor depositionenvironment created in Zone R is a continuum of evaporant fluxes asopposed a step-wise processes. Within Zone R, fluxes are held constantand by translation over the sources the receiving elongate substrateencounters a varying flux of material specifically designed to achieveoptimum performance in the CIGS layer.

Blocks 70, 72, 74, which relate specifically to the vapor-delivery ofcopper, gallium and indium, respectively, represent heated effusionsources for generating plumes of vapor derived from these threematerials. Each of these three effusion source includes: (1) an outerthermal control shield; (2) a boat, reservoir, or crucible containingthe associated molten copper, gallium, or indium; (3) a lid that coversthe associated case and reservoir, and that contains threevapor-ejection nozzles per crucible which assist in creating vaporplumes; and (4) a specially designed and placed heater located near thenozzles. Each such effusion source preferably comprises an elongaterectangular body disposed with its long axis oriented substantiallyorthogonally relative to the direction of strip-material travel inchamber 24.

Directing attention now particularly to FIGS. 16-19, along with FIG. 9,a description now immediately follows which explains the constructionsof effusion sources 70, 72, 74 in more detail. Each of these effusionsources is substantially the same in construction. Accordingly,description now proceeds with reference made specifically (whereappropriate) only to effusion source 70. At the onset of thisdescription of effusion sources construction, we should note that otherspecific effusion sources configurations, with more or less than fourprincipal parts, could be used if desired. The effusion sourceconstruction parameters set forth and referred to herein, should amplyguide those skilled in the art toward the making and using of other,alternative effusion sources structures.

A thermal control shield is disposed external to the crucible and formspart of the effusion source. The shielding consists of two elements, (1)an outer shell, and (2) multiple layers of thermally insulatingmaterials. Function of the outer shell is to restrict motion and hold inplace the multilayer insulation. The shell can be constructed as eithera four wall lid with a top that slides onto the crucible and shieldingor as an four wall rectangular box with a bottom that the shielding andcrucible insert into.

Suitable materials for the shell is limited to materials that cantolerate the high temperature vacuum environment in the presence of hotmetal gases of copper, indium, and gallium, as well as the reactiveselenium. Successfully employed materials for the shell have includedgraphite, boron nitride, tantalum sheet, molybdenum sheet, tungstensheet, rhenium sheet, and titanium sheet. Additional acceptable shellmaterials include the before-mentioned materials coated with aprotective ceramic film such as pyrolific boron nitride, alumina, andtitanium diboride. The material should be chosen to provide sufficientthermal insulation and stability in the reaction zone. A particularlysuitable material is a graphite shell.

Each of effusion source shells 100 has a length dimension herein ofabout 45-cm, a width dimension (measured along the long axis of chamber24) of about 7-cm, and a height dimension of about 7-cm. Length andwidth can be either proportionally or non-proportionally scaled to matchthe zone R and substrate with dimensions, and are matters of choice.From longitudinal centerline-to-centerline of adjacent effusion sources,a preferred distance is about 8.9-cm. Similar to the effusion sourcesize the centerline-to-centerline spacing can be adjusted to the Zone Rdimensions and to the substrate width and are matters of choice.However, from a “plan view” perspective of the effusion source nozzleorganization, this structure is generally centered with respect to thefootprint of zone R.

Moving closer to the crucible is a multi-layer insulation that shieldsthe high temperature sources operating at 1000 to 1700° C. from otheritems in Zone R, including the walls that define Zone R. Among otherthings the most important function of the shielding is three-fold, (1)to reduce the electrical power requirements necessary to maintain thesource at the elevated temperature for extended time periods, and (2) tominimize radiative thermal load and subsequent heating of the surroundcomponents in zone R, and (3) eliminate thermal ‘cross talk’ betweenadjacent sources operating at substantially different temperatures.

Within a vacuum where the key thermal transport mechanism is radiation(as opposed to convection or conduction), effective shielding consistsof several layers, preferably with a low emissivity. Generally, the CIGSeffusion source shielding should offer, (1) thermal stability attemperatures from 1000 to 1700° C., (2) stable and consistent thermalproperties, (3) stability in vacuum in the presence of gaseous metalsand selenium, and (4) stable when in contact with the shell, crucible,and lid materials. Successfully employed materials for the effusionsource shielding have included graphite felt, graphite foil, ceramicfelt, boron nitride sheet, tantalum sheet, molybdenum sheet, tungstensheet, rhenium sheet, and titanium sheet all of which are commerciallyavailable from several vendors. The selected material should providethermal insulation and stability in the reaction zone and multiplelayers of graphite felt and graphite foil have proven particularlysuitable. Three felt layers 101 and nine foil layers 103 in aalternating sequence starting at the crucible of felt, three foils,felt, three foils, felt, three foils provides a suitable specificconfiguration.

Inserted in a somewhat nested condition within the walls of case 100,and formed of pyrolitic boron nitride coated graphite, is an elongateboat, reservoir, or crucible, 102 which is generally rectangular inform, and which includes a base 102 a, a pair of elongate side walls 102b, 102 c which join with base 102 a, and a pair of end walls 102 d, 102e which join with the reservoir base and side walls. Although pyroliticboron nitride coated graphite is the crucible material of choice,several materials are suitable for the crucible including metals oftantalum, molybdenum, and tungsten, several forms of uncoated graphite,and other thermally and vacuum stable ceramics such as alumina, sinteredboron nitride and titanium diboride. Additionally, the crucible can beformed from a combination of the above-mentioned materials coated with athermally stable coating, CTE matched to the crucible material, such aspyrolitic boron nitride, alumina, and titanium diboride. As can be seenespecially in FIG. 17, end walls 102 d, 102 e, as such appear in thisfigure, have thicknesses, of about 0.125-inches, with side walls 102 b,102 c having a somewhat smaller thickness of about 0.1-inches. Base 102a has a thickness of about 0.1-inches. End walls 102 d, 102 e each has apocket-like void space (only one being shown), such as void space 102 fin wall 102 e, which void space is somewhat planar, with the plane ofthis void space being substantially normal to the plane of FIG. 17 Therole of the pocket is to improve the temperature uniformity of themolten metal within the crucible. The pocket improves thermal uniformityby reducing the thermal transport by conduction to the end of thecrucible where heat is radiated to the shielding. Formed in base 102 a,and opening to the outside surface of end wall 102 e, are twolaterally-spaced elongate bores (no reference numbers shown) whichreceive two elongate temperature-monitoring thermocouples such asthermocouples 104, 106 pictured at the lower side of FIG. 18.

The lower end of crucible 102 in FIG. 17 essentially occupies thepreviously-mentioned open end of case 100, through which open end thereservoir can be inserted and removed relative to what can be thought ofas the inside volumetric space provided in case 100. This reservoirincludes an elongate central deep well 102 g that receives and containsmolten copper—such molten copper being indicated at 108 in FIG. 18.

Fitting snugly within the upper portions of previously-mentionedcrucible walls 102 a, 102 b, 102 c, 102 d is an elongate lid 110 formedof graphite. The wall thickness of the material making up lid 110 isabout 0.1-inches. Suitably formed in the upper part of lid 110, i.e., inthat part in the lid that faces the viewer in FIG. 17, and which is nearthe top of FIG. 18, are the three previously mentioned, spacedvapor-delivery nozzles, such as nozzle 112 pictured in FIGS. 17 and 18.The nozzles are essentially unitary with lid 110. In the preferredembodiment, the nozzles are integrally machined into the lid and areformed of graphite, however, nozzles constructed of sintered boronnitride, pyrolitic boron nitride coated graphite, and many refractorymetals such as molybdenum, tantalum, and tungsten have been constructed.Nozzles formed of materials different from the machined lid are insertedinto matching holes in the graphite lid.

The nozzles lie in a common vertical plane which substantially containsthe long axis of vessel 70, with the central nozzle being substantiallycentered relative to the opposite ends of the lid, and the two endnozzles having their axial centerlines each spaced from that of thecentral nozzle by about 15.0-cm. Nozzle spacing can be adjusted based onthe source-to-substrate spacing using methodologies described later inthis disclosure. In brief summary, as the source-to-substrate spaceincreases, the nozzles spacing increases, and conversely, as thesource-to-substrate spacing decreases, the inter-nozzle spacingincreases. Typical inter-nozzle spacing range from about 1 to 20 cm. Thedischarge tips of the nozzles lie preferably in a common plane, whichplane substantially parallels the plane of the path followed by theexpanse of strip material passing through zone R. This nozzle-tip planeis spaced from (above) the substrate-transport plane in chamber 24 from10 to 25-cm but most often at about 18-cm.

Nozzle 112 (typical of all of the crucible nozzles) is shown in anenlarged and more detailed manner in FIG. 19. Here, one can see thatthis nozzle includes an outlet port 114 which is generally cylindricalwith a wall 115, and which possesses an axial length (the verticaldimension of port 114 in FIG. 19) of about 0.95-cm, and a diameter,pictured as the horizontal dimension of the port in FIG. 19, also ofabout 0.95-cm. The discharge openings in the nozzles each preferably hasa diameter within the range of about 0.25-cm to about 2.5-cm, and adepth, measured normal to the plane of FIG. 17, which preferably lies inthe same range. The diameter and depth dimensions are preferably aboutequal.

Extending appropriately into lid 110 from the upper end thereof in FIG.17, and through previously mentioned bores, are two, elongate,conventional electrically-energized heating elements 114, 116,respectively. These heating elements, which are formed preferably ofpyrolithic graphite lie in appropriate slots machined in the graphitelid. Several refractory metals such as tantalum, molybdenum, andtungsten have also been successfully used as the effusion sourceheater(s). As a result of the heater location, the respective lids andnozzles in each of crucibles 70, 72, 74 remain and operate attemperatures which are higher than the temperatures of the associatedmolten materials within the crucible.

The entire effusion source arrangement with the exterior shell,shielding, and spaced exposed nozzles, produces proper deposition of theCIGS materials, while at the same time protecting the strip materialfrom overheating, and substantially eliminates undesired condensation ofmetal vapor liberated from the boats in the regions of the openings ofthe nozzles. The traveling strip material, while directly exposed to theindividual heated nozzles, is shielded from direct exposure to themolten source materials. While two elements are shown herein, more orless in number could be used. For example, a single, generally U-shapedheating element having two long runs leading to a reverse bend withinthe crucible could be employed. U-shaped is the preferred method becauseit allows the heater to be rigidly mounted to the electrical energysource on one end, allowing the other end to freely move as a result ofthermal expansion, in affect, representing a roadway bridge. Hardmounting the electrical source on both ends would likely cause excessivedeformation of the heating element due to thermal expansion, leading topremature failure.

FIG. 10, in dashed lines, shows representative plumes 70 a, 72 a whichemanate from effusion sources 70, 72, respectively, as if to form,nominally, what we sometimes refer to as a vapor-tufted environment,such as was generally mentioned earlier. Use of the word “tufted” hereinis made simply to evoke visualization of how a co-mingleddeposition-vapor fog comes into being in chamber 24. The copper, galliumand indium vapor plumes that exist during CIGS deposition in chamber 24are thought to be a vector quantity in the shape of the form sin² θ asshown in FIG. 32. Effusion from a single orifice source is essentiallythe sum of two processes. The first of these is the evaporation of thesource material, the second is the flow of the vapor through the orificeas pictorially shown in FIG. 31. Each of these processes provides a“resistance” to the effusion of the source material. Evaporation of thesource materials will be described later.

The theory of low-pressure gas flow though an orifice is well understoodand can be predicted to within 5 or 10%. Within a vacuum there are tworegimes in which low pressure gas flow occurs: (1) the free molecularand (2) transitional flow regimes. In qualitative terms, the freemolecular regime describes gas flow in which gas phase collisions arerare enough that only molecule-wall collisions are significant.Transitional flow describes a situation where molecule-moleculecollisions occur frequently enough to affect the flow behavior, but donot occur frequently enough to be described accurately by the fullviscous flow model as would be used at near atmospheric pressure.

The determination of the applicable flow regime is achieved bycalculating the Knudsen number: $\begin{matrix}{{Kn} = \frac{\lambda}{\Gamma}} & \left( {{eqn}.\quad 1} \right)\end{matrix}$

where λ is the mean free path and Γ is the orifice radius. If Kn>1, thesystem is in the free molecular regime and the mass flow rate isdescribed by the following equation: $\begin{matrix}{F_{eff} = {\pi \quad \Gamma^{2}{K\left( \frac{M}{2\pi \quad {RT}} \right)}^{\frac{1}{2}}\left( {p_{1} - p_{2}} \right)}} & \left( {{eqn}.\quad 2} \right)\end{matrix}$

where F is the mass flow rate through the orifice, M is the molecularweight of the gas molecules, R is the ideal gas constant, T is thetemperature, and p₁ and p₂ are the pressures on either side of theorifice. K is an empirically determined constant which is a function ofthe aspect ratio (L/Γ, where L is the orifice length) of the orifice.For L/Γ less than 1.5, K is given by $\begin{matrix}{K = \frac{1}{1 + {0.5\frac{L}{\Gamma}}}} & \left( {{eqn}.\quad 3} \right)\end{matrix}$

For L/Γ>1.5, $\begin{matrix}{K = \frac{1 + {0.4\left( \frac{L}{\Gamma} \right)}}{1 + {0.95\left( \frac{L}{\Gamma} \right)} + {0.15\left( \frac{L}{\Gamma} \right)^{2}}}} & \left( {{eqn}.\quad 4} \right)\end{matrix}$

In the case of 0.01<Kn<1, there are two equations which must be solvedfor both F and p′: $\begin{matrix}{F_{eff} = {\pi \quad \Gamma^{2}{C\left( \frac{M}{2\pi \quad {RT}} \right)}^{\frac{1}{2}}\left( {p_{1} - p^{\prime}} \right)}} & \left( {{eqn}.\quad 5} \right) \\{F_{eff} = {\frac{{\pi\Gamma}^{4}}{16\mu \quad L}\left( {p^{\prime 2} - p_{2}^{2}} \right)\left( {1 + {4\left( {\frac{2}{f_{d}} - 1} \right)\frac{\lambda}{\Gamma}\left( \frac{M}{RT} \right)}} \right.}} & \left( {{eqn}.\quad 6} \right)\end{matrix}$

where μ is the viscosity, f_(d) is the fraction of molecules diffuselyreflected from the walls (0.85<f<1), and C is a constant (C=20).

After determining the mass flow rate, F_(eff), through the orifice itbecomes necessary to describe the flux intensity profile of the effusingbeam, that is, to determine f=f(r, θ), where f is the flux, r is thedistance from the effusion orifice, and θ is the azimuthal angle.

An equation describing the flux as a function of θ and the rate ofeffusion is obtained by setting the rate of effusion equal to theintegral of the flux over a hemispherical area. Assuming that the fluxcan be approximated by f=a cos^(n) θ $\begin{matrix}{F_{eff} = {\int_{0}^{\frac{\pi}{2}}{\int_{0}^{2\pi}{a\quad \cos^{n}{\theta \left( {r^{2}\sin \quad \theta} \right)}\quad {\partial\xi}{\partial\theta}}}}} & \left( {{eqn}.\quad 7} \right)\end{matrix}$

After solving for a in eqn. 7, $\begin{matrix}{f = {\frac{F_{eff}\left( {n + 1} \right)}{2\pi \quad r^{2}}\cos^{n}\theta}} & \left( {{eqn}.\quad 8} \right)\end{matrix}$

Although the a priori prediction of a value for n is not completely welldefined, a safe approximation for both transitional flow regimes andfree molecular regimes of L/D=1 is n=2. The molecular beam profile isdepicted in FIG. 32.

Effusion rate from a given nozzle is a function of vapor pressure withinthe inside of the associated crucible, and that this pressure is afunction of the temperature of the molten material inside the reservoirin that crucible. Thus, for a particular, selected nozzle size, theeffusion rate to be expected is essentially a function of thetemperature within the crucible.

Predicting the rates of effusion of the copper, gallium, and indiumsources is a straightforward solution of the equations above. Thetemperature-vapor pressure data of the three elements are easily foundin literature and can be approximated by:

Cu: log P _(Cu) ^(sat)=−19.818+2.0643×10⁻² ×T−5.2119×10⁻⁶ ×T ²  (eqn. 9)

Ga: log P _(Ga) ^(sat)=−17.2982+2.0829×10⁻² ×T−6.0×10⁻⁶ ×T ²  (eqn. 10)

In: log P _(In) ^(sat)=−16.238+2.1427×10⁻² ×T−6.7885×10⁻⁶ ×T ²  (eqn.11)

where pressure is in torr and temperature is in °C.

FIGS. 24-26, graphically picture the respective effusion rates ofcopper, gallium and indium that we have observed with respect to nozzlesconstructed in accordance with the descriptive information given above.These graphs relate to effusion rate (grams-per-hour for differentmolten-material temperatures) with respect to the activity of a singlenozzle. In particular, FIGS. 24-26 illustrate the effusion rate ofcopper, gallium, and indium as a function of temperature for an orifice0.9525-cm in diameter and 0.9525-cm in length. The copper, indium, andgallium sources all operate in the free molecular flow regime. It isalso noteworthy that the evaporation process in these cases isresponsible for <5% of the resistance of the total effusion process.That is, the orifice geometry is the overwhelming factor in determiningrate of effusion.

Further application of the above principals reveals that the vapor fluxincident at the deposition surface presented in chamber 24 is,essentially, a function of the temperature within a selected crucible,the distance between that crucible and the intended deposition surface,and the angle between a point on the substrate and the effusion sourcenozzle. Accordingly, it will be apparent that, for a fixed distancebeing decided upon to exist between the crucibles and the travelingstrip material in chamber 24 traveling at a constant speed, the amountof metal vapor (collectively) incident at the deposition surface of thetraveling strip material is essentially a function of the temperature ofthe molten materials within the crucibles.

Thus, it should be very apparent, that, by carefully controlling thetemperatures of the molten materials within crucibles 70, 72, 74, and bymaintaining substantially constant the transport or travel speed of thestrip material in chamber 24, the rate at which metal vapor from eachcrucible is applied to the appropriate deposition surface of thetraveling strip material can be controlled readily to produce uniformthin-layer deposition thickness along the length of such material.

In accordance with a preferred embodiment of the system of the presentinvention, the molten temperature of copper within crucible 70 issuitably maintained in the range of about 1400° C. to about 1700° C.,and most preferably at a temperature of about 1565° C. (±about 1° C.).The temperature of the molten gallium within crucible 72 is suitablymaintained within the range of about 1000° C. to about 1350° C., andmost preferably at about 1225° C. (±about 1° C.). Finally, thetemperature of the molten indium within crucible 74 is mostappropriately (according to what we have learned in our practice of useof this invention) maintained in the range of about 950° C. to about1300° C., and most preferably at about 1205° C. (±about 1° C.). Thetemperature of molten selenium in reservoir 85 b is preferablymaintained in the range of about 275° C. to about 500° C., and mostpreferably to about 415° C. (±about 10° C.). Although these temperaturesare in the preferred range for the disclosed sources with 0.95-cmorifices, the rate restricting nozzle principals outlined hereinindicated that as the orifice size increases, the temperature woulddecrease to achieve a constant rate, or alternatively, as the orificedecreases, the temperature would increase to achieve a constant rate.

It should be noted that, although the effusion rate (and hence the flux)of selenium vapor is quite sensitive to changes in temperature in thebody of molten selenium in reservoir 85 b, changes in selenium flux overtime do not appreciably affect the formation of the end-result CIGS/CISlayer because the chamber is already essentially saturated withselenium.

By carefully controlling the vapor effusion rates from the nozzles incrucibles 70, 72, 74, and from those in the sparger tubes, for exampleby proper dimensioning of the respective collections of nozzles, andfurther by carefully controlling the temperatures of the molten metalswithin the reservoirs in the crucibles and in the selenium deliverystructures, and thereby effectively controlling the pressures withinthese crucibles and the “selenium structures”, the desired effusioncharacteristics of the generated vapor plumes may also be carefullycontrolled. Coupling to these considerations, the further considerationsof (1) selecting an appropriate number of nozzles for eachvapor-delivery crucible and sparger tube, (2) appropriately positioningthe nozzles in the lids of the respective crucibles and in the spargertubes, and (3) carefully arranging the overall disposition layout of thecrucibles and sparger tubes, an optimum aggregate multiple-plumeconfiguration can be obtained.

The particular positionings and sizings of the nozzles present inchamber 24 as described herein, with each of the nozzles havingsubstantially the same configuration as each other nozzle, produces apreferred arrangement in chamber 24 for the deposition of our desiredCIGS/CIS layer. From a reading of plume geometry principles describedherein, coupled with a careful review of the specific designconsiderations so far described herein for the layouts of the cruciblesand their nozzles, those skilled in the art will see that carefulselection of the size and number of orifices for each such kind ofcrucible can produce substantially any desired, and substantiallytransversely uniform, vapor flux across the width of a depositionsurface, which uniformity will result in a substantially uniform layerthickness throughout the resulting deposited thin-film layer.

The flux seen at a surface is a function of both the intensity of theincident flux and angle of incidence. As the angle of incidence, φ (heredefined as the angle between the surface normal vector and flux vector),increases, the deposition flux seen by the surface decreases as thecosine of the angle of incidence.

f _(dep) =f cosφ  (eqn. 12)

Combining equations 11 and 12, we obtain an expression for thedeposition flux at a point on a surface due to a single source:$\begin{matrix}{f_{dep} = {\frac{F_{eff}\left( {n + 1} \right)}{2\pi \quad r^{2}}\cos^{n}{\theta cos\varphi}}} & \left( {{eqn}.\quad 13} \right)\end{matrix}$

If the centerline of the source orifice is parallel to the surfacenormal vector (i.e. the source is not tilted—See FIG. 33), eqn. 13reduces to $\begin{matrix}{f_{dep} = {\frac{F_{eff}\left( {n + 1} \right)}{2\pi \quad r^{2}}\cos^{n + 1}\theta}} & \left( {{eqn}.\quad 14} \right)\end{matrix}$

Knowing the molecular weight, MW, and density, ρ, of the material beingdeposited, the rate of growth, δ, (thickness/time) at a point on thesubstrate surface is written as: $\begin{matrix}{\delta = {\frac{F_{eff}\left( {n + 1} \right)}{2\pi \quad r^{2}}\cos^{n + 1}\theta \times \frac{MW}{\rho}}} & \left( {{eqn}.\quad 15} \right)\end{matrix}$

By applying the equations outlined above, the cumulative fluxdistribution of several nozzles within a single source can bedetermined, and subsequently the nozzle spacing can be determined asdiscussed early in the document during the detailed disclosure of theeffusion source dimensions. FIG. 34 graphically pictures the respectiveeffect of overlapping plumes for two and three nozzles within a 15-cmspan of a single effusion source. This graph shows the optimum nozzlespacing is approximately 15-cm as disclosed earlier.

Further regarding these nozzles/discharge openings, under certaincircumstances, it may be desirable to construct and employ, for one ormore of the several deposition materials, discharge orifices which havedifferent discharge diameters or shapes. In other words, a given vesselor boat for a particular deposition material may have associated with itplural orifices with different discharge diameters or shapes. Orificedischarge diameter plays an important role, inter alia, in definingcertain aspects (for example, vapor-discharge volume per unit of time)of its associated discharge vapor plume.

Those skilled in the art will recognize, accordingly, that certainaspects of discharge behavior can be adjusted to suit particular needsand circumstances through suitable adjustments made in nozzlecharacteristics. In addition, it is possible to have the effusionsources supply materials in a downward, lateral or oblique orientationto the substrate, as shown in FIGS. 27-29.

It should be noted that, although the disclosed embodiment utilizesseparate chambers, it is also possible to combine two or more of theprocessing steps into a single chamber. However, separate chamberprocessing offers a number of advantages. The layers of CIGS (Mo, CIGS,CdS, i-ZnO, and ITO) consist of several very mobile atoms when in agaseous form in a vacuum where the mean free path is long, especiallyselenium, sulfur, cadmium, zinc, and tin. This mobility is primarilyrelated to high vapor pressures even at moderate temperatures.Cross-contamination of one element in another processing zone candramatically alter (negatively) the properties of the layer. In in-lineglass lines, several isolation lock chambers are used to prevent crosscontamination. An individuated plate of glass is moved into the lockfrom the upstream deposition zone while a valve isolates the lock fromthe downstream processing zone. Once the plate is fully in the zone, thevalve is closed between the lock and the upstream zone, the vacuumenvironment is stabilized to the downstream zone, and then the valve tothe downstream zone is opened. The lock chambers usually also containspecial provisions to preferentially pump constituents (i.e., Se, water)that reside in the processing zones on either side of the lock chamber.This approach is not technically practical with a fully continuous rollto roll process, especially one in which only the backside of the web iscontacted with rollers. It is done in some instances when applying hardcoatings to flexible substrates but these systems usually contain niprolls or thin membranes that drag on the coated side of the substrate tocreate a seal (i.e., the back of the strip is sealed with a roller andthe front is sealed with a roller or dragging membrane. However, it ispreferable not to contact both sides of the web.

Separate chambers are also beneficial because of the substantialdifferences in environment between separate zones, i.e., Mo is typicallydone at ˜1.5 mtorr with argon and water, CIGS is typically done at 0.001mtorr with no argon or water, CdS is typically done at 2 mtorr with justargon. With a combined chamber, a lock is used to stabilize theenvironment before the valves separating the zones from the lock isopened. For example, as the lock chamber is brought to the pressure ofthe Mo zone, the valve separating the Mo zone from the lock is opened.Bringing the two zones to the same pressure is important to prevent apressure surge in the Mo zone. The valve between Mo and the lock isclosed, the lock is pumped to the pressure of the CIGS processingchamber, and then the valve between the lock and the CIGS zone isopened. Such an arrangement adds considerable unnecessary complexity tothe system.

Another advantage of separate chamber processing is that downtime forunscheduled maintenance for repair does not take down the entire plant.This is particularly important where there are several chambersinstalled for each layer because even if one CIGS is down, for example,the entire plant is not down. In addition, scheduled downtime for sourcereplenishment does not require stopping an entire line. It is alsoeasier to scale the processing to the desired production rate becausethe chambers can be independently sized, i.e., not all chambers have torun at the same rate. For example, a single Mo chamber may be able toprocess web fast enough for four CIGS chambers, two CdS chambers and twoITO chambers. Similarly, additional chambers can be added more easily.

Chamber 24 is preferably maintained at a pressure of about 5×10⁻⁵-Torr.Tension within the straight part of the transported moving strip(between rolls 60, 68) is held typically to within a range of about 0.5-to about 20-kg, and most preferably to within a range of about 3- toabout 4-kgs. Linear transport speed lies preferably within the range ofabout 15-cm-per-minute to about 2-meters-per-minute, and most preferablyabout 30-cm-per-minute.

As strip material travels in chamber 24, from left-to-right as indicatedby arrow 16, the developing CIGS layer increases generally quitelinearly in thickness, from zero at the entrance end of deposition zoneR to (preferably) within the range of about 1- to about 3-micrometers,and most preferably to about 1.7 to 2.0-micrometers, at the downstream,exit end of this zone.

Zone R (the fog-containing zone) is illustrated in FIG. 10 as afreestanding rectangular block. Zone R has a length Z herein (FIGS. 6,7, 9 and 10) of about 10- to about 250-cm, and most preferably about80-cm, a width W (FIGS. 6, 7, 9 and 10) of about 90-cm, and a height H(FIGS. 6, 7 and 10) of about 25-cm. All Zone R dimensions are matters ofchoice and are somewhat a function of the substrate strip width and theeffusion source size, and as mentioned before effusion source design andconstruction techniques outlined in this patent can be universallyapplied to very small strip widths and effusion source sizes, (i.e.,2.5-cm wide substrate strip and 5-cm wide sources) to very largereaction zones and substrate strip widths (1.5-m wide substrate stripand 2-m wide effusion sources). The fog in zone R is effective, as stripmaterial passes through the zone, to create an extremelyuniform-thickness, controlled-content, multi-element CIGS layer, such aspreviously mentioned layer 38.

Referring to FIG. 6, circles 76, 78, 79, 81 represent end views ofplural, laterally spaced, generally parallel elongate sparger tubes (orfingers) that form part of a comb-like, single manifold (see also FIG.9) that supplies, to the deposition environment within chamber 24, arelatively evenly volumetrically dispersed selenium vapor. Each spargertube has a length of about 30-inches and a diameter of 0.25-in, andadjacent pairs of these tubes are characterized by a tube-to-tubespacing of about 8.9-cm. Length and spacing of the sparger tubes are,similar to the effusion sources, material of choice based on thesubstrate strip width and the effusion source size. Again principalsoutlined in this disclosure are applicable regardless of scale. Eachtube, as illustrated herein, has three linearly spaced and distributedoutlet orifices or vapor-ejection nozzles that are spaced from oneanother by about 15-cm. The diameter of each such sparger-tube orificeis about 0.1-cm. The “collection” of sparger-tube orifices (from a planpoint of view) is substantially centered on Z×W footprint of zone R, andthese orifice's discharge (upper) ends lie generally in a common planewhich substantially parallels the strip-material transport plane at adistance of about 17.8-cm.

Four such sparger tubes are employed in the chamber structure now beingdescribed, with each sparger tube being equipped three vapor-ejectionnozzles, such as representative nozzle 76 a in sparger tube 76 (seeparticularly FIGS. 9, 16 and 20). Nozzle 76 a has a diameter herein ofabout 0.100-cm_(the nozzle's vertical dimension in FIG. 20), and anaxial length of about 0.71-cm (its horizontal dimension in FIG. 20).Significantly, the delivered selenium vapor, which resides essentiallyat the saturation point within chamber 24, is derived from a singlepool, site, or reservoir 85 b (shown in FIGS. 9 and 15) of moltenselenium.

These tubes are preferably made of stainless steel, but can beconstructed of any material that is stable at high temperature, invacuum in the presence of selenium and metal vapors. The tubes arelocated generally as shown in positions effectively bracketing opposite(left and right) sides in FIGS. 6 and 9 of each of blocks 70, 72, 74.This special selenium-vapor distribution system is preferable to priorart systems, which typically employ a plurality of spaced molten poolsof selenium. Heat which functions to trigger and to sustain appropriatedownstream vapor-distribution operation of this selenium-delivery systemis derived from the close proximities of the sparger tubes and theheated crucibles in structures 70, 72, 74. In other words, radiant heatfrom these crucibles plays an important role in the delivery ofselenium.

It should be noted that crucibles of structures 70, 72, 74, spargertubes 76, 78, 79, 81, and the respective nozzles associated with thesestructures, are collectively substantially centered on the footprint ofzone R (its W and Z dimensions). Preferably, the width W of zone R issomewhat greater than the width of the traveling strip material, andlaterally, the width dimension of the strip material is substantiallycentered on width W. Consequently, the lateral boundaries, or edges, ofthe strip material are completely within zone R, and as strip materialpasses through the zone, it is treated to a generally bilaterallysymmetrical engagement with the vapor components being deposited. Thebilateral symmetry just mentioned is such symmetry viewed relative tothe long axis 24 a of chamber 24. See FIG. 9.

In general terms, and as is pictured schematically by the threedownwardly-curving dashed lines in FIG. 6 that represent billowingplumes (such as previously mentioned plumes 70 a, 72 a) of copper,gallium and indium, created within chamber 24, generally in thepreviously mentioned deposition region, or zone, R, is a specialco-evaporation fog (mentioned earlier) which is formed from theseseveral effusion plumes (copper, gallium and indium), and from theselenium vapor (the sparger tubes) mentioned above.

At substantially each longitudinal point or transverse slice along zoneR, and as a result of the nozzle spacing determined earlier, the fogtherein is effectively uniform across the width of the zone, i.e., alongthe direction transverse to the direction of strip-material travel. As aresult, each point along every line extending across the width of thestrip material (perpendicular to the direction of material travel)advantageously is subject to approximately the same material-specificflux from each material-specific boat at any particular instant.

As the strip of Mo-coated substrate material travels in chamber 24through the vapor deposition zone, each point on that material firstpasses directly over the copper source, thereafter over the galliumsource, thereafter over the indium source, and throughout, over theselenium sources (the sparger tubes). With this arrangement, andrecognizing that selenium vapor is distributed rather evenly throughoutzone R, each point on the moving strip material first encounters acontinuum co-evaporation environment which is substantially copper-rich,but which also contains lesser amounts of gallium and indium vapor. As aparticular point on the strip travels through the fog, it nextencounters a region, in the commingled, aggregate fog, which issubstantially gallium-rich, but which also contains copper and indiumvapor. Each point on the strip thereafter encounters a region in zone Rthat is substantially indium-rich, but which contains lesserconcentrations of gallium vapor and copper vapor. Significantly andpreferably, this transition from copper-rich, through gallium-rich andfinally to an indium-rich vapor occurs in a setting wherein thespecific, individual fluxes from molten copper, gallium, indium, andselenium sources are maintained substantially constant as a function oftime. In the process of the invention now being described, it is thetravel of the strip material through zone R that causes each point onthat material to experience the aforementioned,longitudinally-spatially-changing vapor (fog) sub-environments. Bymaintaining the effusion rate from each boat substantially constant overtime, and by moving the strip material at a substantially constant rateof speed, the contribution to the CIGS layer attributable to each of themolten metal sources may be precisely controlled along the length of thedeposition zone. Using similar equations applied to determine theoptimum nozzle spacing within a source, as the strip transitions throughthe zone, at any instantaneous point in time, the instantaneous flux atthe strip and the cumulative composition of the CIGS film deposited onthe strip can be determined. The strip will have a incident flux of eachof the elements that is a strong function of the position of the stripin the zone. The incident flux at any instantaneous time can be plottedas the strip transfers through the zone. FIG. 35 shows the instantaneousflux as a function of position as the web transfers through Zone R forthe copper, gallium and indium sources. This figure shows that the stripfirst encounters a large copper flux, but also a flux from the galliumand indium. As the strip continues, the gallium, and then the indiumfluxes become dominant.

In a similar fashion, the cumulative composition of copper, gallium, andindium can be calculated for the strip as it transitions through thezone, as graphically shown in FIGS. 36 and 37. As was mentioned above,the respective vapor effusion rates of copper, gallium and indium fromthe crucibles/boats in structures 70, 72, 74, respectively, arecontrolled in such a fashion that the entrance end of zone R iscopper-rich, the middle region of this zone is gallium-rich, and theexit end of the zone is indium-rich. In particular, we have found that,by establishing appropriate effusion rates for copper, gallium andindium: (a), within the entrance end of zone R, the ratio (Cu)/(Ga+In)is generally about 3.4, and the ratio (Ga)/(Ga+In) is generally about0.46; (b), within the middle region of zone Z, the ratio (Cu)/(Ga+In) isgenerally about 1.9, and the ratio (Ga)/(Ga+In) is generally about 0.43;and (c), within the exit end of zone Z, the ratio (Cu)/(Ga+In) isgenerally between 0.8 and 0.92, most preferably, about 0.88, and theratio (Ga)/(Ga+In) is between generally between 0.25 and 0.3, mostpreferably 0.275.

The CIGS layer created with either chamber organization (FIGS. 6 and 12)has an internal make-up or composition of approximately 23.5 atomicpercent copper, 19.5 atomic percent indium, 7 atomic percent gallium,and 50 atomic percent selenium. The key difference is the CIGS formed inthe chamber of FIG. 12 better tolerates mechanical stresses imparted onthe film as a consequence of fabricating the unique flexiblephotovoltaic device disclosed herein.

As described above, the copper, gallium, and indium effusion sourceseach include a shielded, insulatively enclosed, heated subchamber orcrucible wherein source material is heated to a molten condition, andfrom which subchamber vapor-phase metal readily exits through the nozzleopenings to become part of the deposition fog. As a result, the vesselsmay be placed fairly close to one another and fairly close to the pathof the traveling strip material. This allows precise control over thedistribution geometry of the respective plumes emanating from eachrespective effusion source and, therefore, effective control over theaggregate fog resulting from the commingling of the plumes from allthree effusion sources. In addition, by so establishing thermalinsulation and isolation, the effusion sources may be placed closeenough to the path of strip travel to maximize (as suggested above) theeffectiveness of material deposition without causing overheating of thereception strip material.

During co-evaporation in chamber 24, and in accordance with practice ofthis invention, special attention is directed toward the production andmaintenance of the effective deposition temperature, also called hereinthe local processing temperature, of the strip-material surface uponwhich deposition occurs. To assure proper interlayer integrity andadhesion between Mo and CIGS and to assure proper formation of anultimately well-functioning, clearly defined and establishedpolycrystalline CIGS layer, it is preferable that the local spot orregion which is currently receiving deposition treatment be held atbetween 300 and 650° C., preferably below 450° for the polyamidesubstrate and at 550° for substrates capable of higher temperatures suchas stainless steel, titanium, and glass.

From all of the above discussion relating to nozzle placement,population and sizing, it is apparent that there is substantial room tovary any one of more of these parameters to achieve a deposition fogenvironment of a specific desired character. Thus, while we have foundpreferable for the specific process described in this document to havethree nozzles present in each of crucibles 70, 72, 74, and with eachnozzle in each crucible being substantially the same in construction(sizing, etc.), these particular choices could be changed. For example,one could choose to use more or less than three nozzles per crucible.One, also, could choose to use different numbers of nozzles with respectto different crucibles. The nozzles themselves, (either with regard toan inter-crucible way of thinking about things, or with regard to anintra-crucible way of thinking about things), could have differentrespective axial lengths and diameters.

For the purpose of describing one preferred way of creating a CIGS layerherein, chamber 24 is illustrated and specifically discussed asincluding (in addition to the structure provided for delivering seleniumvapor) just the three vapor-delivery blocks 70, 72, 74. However, a veryuseful alternative approach employable in practicing this inventionemploys a multitude of such blocks distributed within chamber 24. Use ofthis alternative allows unique control of the CIGS through thicknesscomposition that can alter, positively or negatively, the performance ofthe resultant CIGS thin film photovoltaic material.

Several source orders have been investigated to achieve the optimumcomposition. An alternative construction for chamber 24 is illustratedin FIGS. 12-14. The primary distinction, relative to the first-describedchamber-24 construction, is the use of five rather than three elongate,heated, vapor-delivery effusion sources 86, 88, 90, 92 and 94 (generallylike previously discussed effusion sources 70, 72, 74). It should benoted that a space with no vapor source is provided between second andthird effusion sources. The nozzles (three each) in the central vessel90 delivers copper vapor; those in effusion sources 86 and 92 deliverygallium vapor; and those in vessels 88 and 94 deliver indium vapor. Anadditional modification includes a final gallium deposition employedafter the final indium in FIGS. 12-14, more will be said about thisalternative possibility later—a special alternative which can be thoughtof as possessing “longitudinal vapor (material)-delivery symmetry”.

Thus, each point on the surface of a strip passing through this versionof chamber 24 encounters, in sequence: a gallium/indium-rich region, acopper-rich region, and finally, another indium/gallium-rich region.This “encounter” experience can be thought of as involving a kind oflongitudinal material-deposition symmetry within zone R. Similar to thethree source example, instantaneous flux of a point on the strip can beplotted as a function of position in Zone R, as graphically shown inFIGS. 38 and 39. As was mentioned above, the respective vapor effusionrates of the five effusion sources 86, 88, 90, 92 and 94 respectively,are controlled in such a fashion that the entrance end of zone R isindium-gallium, the middle region of this zone is copper-rich, and theexit end of the zone is gallium-indium-rich, as graphically illustratedin FIG. 40. In particular, we have found that, by establishingappropriate effusion rates for all sources: (a), within the entrance endof zone R, the ratio (Cu)/(Ga+In) is generally about 0.0, and the ratio(Ga)/(Ga+In) is generally about 0.35; (b), within the middle region ofzone Z above the copper source, the ratio (Cu)/(Ga+In) is generallyabout 1.1, and the ratio (Ga)/(Ga+In) is generally about 0.2; and (c),within the exit end of zone Z, the ratio (Cu)/(Ga+In) is generallybetween 0.8 and 0.92, most preferably, about 0.88, and the ratio(Ga)/(Ga+In) is between generally between 0.25 and 0.3, most preferably0.275.

In accordance with a preferred embodiment of the system of the presentinvention, the molten temperature of gallium within crucible 86 issuitably maintained in the range of about 1000° C. to about 1350° C.,and most preferably at a temperature of about 1120° C. (±about 1° C.).The temperature of the molten indium within crucible 88 is suitablymaintained within the range of about 950° C. to about 1300° C., and mostpreferably at about 1130° C. (±about 1° C.). The temperature of themolten copper within crucible 86 is suitably maintained within the rangeof about 1350° C. to about 1700° C., and most preferably at about 1496°C. (±about 1° C.). The temperature of the molten gallium within crucible92 is suitably maintained within the range of about 1000° C. to about1350° C., and most preferably at about 1130° C. (±about 1° C.). Finally,the temperature of the molten indium within crucible 94 is mostappropriately (according to what we have learned in our practice of useof this invention) maintained in the range of about 950° C. to about1300° C., and most preferably at about 1055° C. (±about 1° C.), Thetemperature of molten selenium in reservoir 85 b is preferablymaintained in the range of about 275° C. to about 500° C., and mostpreferably to about 415° C. (±about 0° C.). As stated before, althoughthese temperatures are in the preferred range for the disclosed sourceswith 0.95-cm orifices, the rate restricting nozzle principals outlinedherein indicated that as the orifice size increases, the temperaturewould decrease to achieve a constant rate, or alternatively, as theorifice size decreases, the temperature would increase to achieve aconstant rate.

This sequential exposure to the initial copper-poor material, followedby copper-rich material as the strip passes over the central coppersource, and then transition back to a slightly copper-poor CIGScomposition, results in a CIGS film with a good combination of adhesionto the underlying Mo layer and high conversion efficiency. Variations ofthe sequence between the gallium and indium sources on both sides of thecentrally located copper have also been employed successfully.

Essentially all other environmental and operational conditions andparameters within chamber 24, as pictured in FIGS. 12-14, as well as alltemperatures, spacing and other dimensions, match substantially thosecounterpart characteristics present in the first-discussed version ofchamber 24. The chief apparent difference resides in the distributedorder and pattern in and by which the positional-content-variableconstituents of the deposition fog are encountered by regions on thetraveling strip material. CIGS/CIS deposition by way of an arrangementsuch as that pictured in FIGS. 12-14, offers somewhat differentopportunities for deposition delivery control than does the arrangementshown in FIGS. 6-10.

Differentiating reasons for choosing to employ, for example, one or theother of these two illustrated arrangements are outlined below. For thethree source arrangement, principal advantages include simplifiedcontrol of three sources, ensuring the copper rich stage which is animportant factor in achieving high efficiency, however, controllingadhesion of the CIGS deposited by the three source technique tosubstrates with a large coefficient of thermal expansion mismatch withthe absorber is difficult. Additionally, the three source arrangement ismore likely to lead to through thickness composition variations that maylimit the photovoltaic device performance. Principal advantages of thefive source arrangement are improved adhesion, even with substrates witha large CTE mismatch to CIGS, and greater ability to tailor the throughthickness composition to enhance the photovoltaic device performance.The key challenge with the five source arrangement is controlling therelative fluxes of indium and gallium on either side of the centrallylocated copper source 90. We further recognize that other considerationsmight well dictate a preference for selecting and implementing analtogether different distributed vapor-plume layout, and these otherkinds of approach can certainly be determined easily by those skilled inthe art in view of the present disclosure.

Typically, the exposed surface of a deposited CIS or CIGS layer willhave a quite irregular, three-dimensional surface topography, with suchtopography being characterized by many randomly distributed peaks andvalleys. The character of this surface is the primary determinant of thedesirability of utilizing an i-ZnO layer immediately underneath thefinal, conductive ZnO:Al layer. It is important that the CdS layercompletely separates the CIS layer from the top contact layer.Therefore, where the CdS layer alone provides adequate separation, nosupplemental insulating layer is necessary. However, where the CdS layerdoes not sufficiently cover the CIS layer, the i-ZnO layer can providesuch separation. As was mentioned earlier, the present invention avoidsthe necessity of using a CdS wet-dipping technique—favoring instead, andpreferably, the application of the CdS layer by way of RF-sputtering.However, the resulting CdS layer is thinner and less certain tocompletely isolate the two layers it lies between. This issue is moresignificant where a typical, prior art, wet-dipping technique forapplying a layer of CdS is not used. With RF-sputtering preference, itis generally desirable, at least in certain instances, to include suchan i-ZnO intermediary layer. Determination, of course, about whether toinclude, or not to include, this layer is dependent on the particularapplication and is a matter of design choice.

The chamber-representing blocks 26, 28 and 30 illustrated in FIG. 11 canbe viewed, as has been mentioned, as illustrating the steps, and theequipment employed therefore, involved in creating (a) the mentioned CdSlayer, (b) the mentioned optional i-ZnO layer, and (c) the finalconductive-oxide ZnO:Al over-layer.

The short open arrow at the left side of FIG. 11 represents inputparameters and equipment related to the deposition environment whichexists within the processing chamber drawn in FIG. 11. The particularsof such input parameters are specifically related to the specific taskto be performed in a chamber like the one drawn in FIG. 11—i.e., CdSdeposition, i-ZnO deposition, and ZnO:Al deposition.

Viewing FIG. 11 first of all as an illustration relating to CdSdeposition, and with the chamber representation for this purpose beingnumbered 26, appropriate equipment is provided in and for this chamberto implement a roll-to-roll procedure for the formation, on thepreviously formed CIGS layer, of a CdS layer. In the practice of thepresent invention, the CdS layer is created efficiently, inexpensivelyand safely in chamber 26 by way of RF-sputtering. Preferably, suchsputtering is used to create a CdS layer with a thickness generally inthe range of about 300- to about 2500-Angstroms. Most preferably, in acase where an intermediary i-ZnO layer is employed, the CdS layer has athickness of about 600-Angstroms. In a situation where such anintermediary layer is not used, the CdS layer may have a thickness ofabout 1200-Angstroms. This sputtering approach to the building of theCdS layer is effective at creating substantially full-surface coverageof the underlying CIGS layer—i.e., dealing with the many typical peaksand valleys mentioned earlier which exist on the exposed surface of theCIGS layer. It should be noted that low frequency alternating current,or AC, sputtering could also be employed to deposit the CdS layer.

RF-sputtering of the CdS layer employs an appropriate CdS target, andthis sputtering takes place preferably at an RF frequency of 13.5-MHz.The power level employed for sputtering is chosen to coordinatelayer-formation activity with a selected strip linear transport speed toachieve the desired CdS layer thickness. An appropriate transport speedlies in the range of about 1.0-cm-per-minute to about2-meters-per-minute, and a transport speed of about 30-cm-per-minute isparticularly suitable. Within this strip transport speed, appropriate RFpower ranges are between 100 and 1200 watts, most appropriately at 300watts.

Following CdS-layer formation, the take-up roll 60 from chamber 26 whichnow contains a layer structure including CdS is transferred to anotherisolated processing chamber 28 of FIG. 11, wherein the optionalintermediary i-ZnO layer is created. This layer is established utilizinga DC sputtering technique to achieve a final layer thickness preferablyin the range of about 100- to about 1000-Angstroms, and most preferablyabout 400-Angstroms.

Referring now specifically to the making of this i-ZnO layer, oxidetargets employed for this purpose are typically sintered duringmanufacture, and during such sintering, these targets often lose some oftheir elemental oxygen, thus rendering the target substoichiometric.This loss of oxygen and concomitant substoichiometric condition rendersthe target slightly conductive, as opposed to a stoichiometric zincoxide target which would be nonconductive.

Recognizing the fact that, where an i-ZnO sublayer is to be created itshould end up as a very poorly conductive layer, we neverthelesspreferably choose to employ substoichiometric zinc-oxide as a “starter”material because such material especially facilitates the use of thepreferred DC sputtering technique, as opposed to an RF sputteringtechnique. In this context, the use of such a substoichiometric (e.g.,oxygen-deficient) zinc-oxide target tends to suppress and even eliminatethe usual positive charge buildup which tends to occur on the surface ofmore conventional, stoichiometric targets during DC sputtering. In thepresent invention, the substoichiometric starter target is compensatedfor by, for example, providing a supply of oxygen into the chamber-28sputtering environment. In particular, an appropriate external supplytank containing compressed oxygen can be used to furnish and support acontrolled bleed of oxygen into chamber 28. By providing oxygen intosuch a sputtering environment, the substoichiometric zinc-oxide targeteffectively applies an appropriate conductivity intrinsic-zinc-oxidelayer to the traveling strip material, notwithstanding the target'snominal, oxygen-deficient, substoichiometric starter character. Itshould be noted that RF or low frequency alternating current, or AC,sputtering could also be employed to deposit the i-ZnO layer usingstoichiometric ZnO targets.

The last PV-operative layer to be created according to the practice ofthe present invention is the overlying conductive-oxide layer, hereinZnO:Al. This is done in chamber 30 (the third point-of-view for FIG. 11)under appropriate internal environmental conditions which are effectiveto create a final ZnO:Al layer with a thickness in the range of about2000- to about 15,000-Angstroms, and most preferably within the somewhatnarrower range of about 10,000- to about 12,000 Angstroms.

The next steps comprise the methodology for monolithically integratingindividual PV cells on the elongated strip from above into a PV module.

Following completion of the i-ZnO deposition or alternatively, CdSdeposition where the optional i-ZnO layer is not utilized, the roll istransferred to the laser isolation equipment. The laser is utilized toselectively remove material (e.g. CIGS or CIS, CdS, intrinsic ZnO, andMo) through the Mo layer to the PI, thereby electrically isolatingadjacent cells. In the present embodiment, the beam of laser light moveswhile the substrate is stationary, but other options can presumably beas effective. For this step the laser presently moves relative to thesubstrate at 12-inches per second.

Using the same equipment, a dielectric layer, which utilizes a UVcurable insulating polymer, is concurrently applied by ink-jet printingmethods over the laser-scribed area, and within seconds is cured by adirected, intense UV source. The UV curable material was chosen suchthat it can be cured rapidly after application and such that it can beapplied over a width less than 400 micrometers and preferably less than125 micrometers but greater than 60 micrometers. The orifice diameter ofthe ink-jet applicator is between 100 micrometers and 300 micrometerswith a preferred diameter of approximately 100 to 150 micrometers. Thethickness of the resulting layer is approximately 60 micrometers,however this specific dimension is not considered critical. The moresignificant properties for this UV curable insulating material are itsadhesion to the CdS, the i-ZnO, if utilized, the CIGS or CIS, and theMo, its viscosity, its slump, its rate of cure, and its surfacecharacteristics since it must allow for good over-layer adhesion by theconductive oxide. Various combinations of these critical parameters willresult in satisfactory performance.

Again using the same laser equipment, a second laser cut is utilized forthe selective removal of material (e.g. CIGS or CIS, CdS, intrinsic ZnO)to provide access to the Mo layer. This access is necessary to allow thenext layer, the conductive-oxide overlayer, to bridge from the top ofone cell to the back electrode (Mo) of the adjacent cell. For this stepthe laser presently moves relative to the substrate at 30-cm per second.The conductive-oxide overlayer is deposited as described earlier. Athird laser cut is utilized for a selective removal of conductive-oxideoverlayer material, thereby isolating individual cells and completingthe monolithic interconnection of previously-individual adjacent cells.

Assuming that proper thin-film “patterning” has been performed to resultin monolithically interconnected PV modules, then protectiveovercoatings, if any such coatings are desired, are then produced in aconventional manner, and electrical contact structures, which arerequired to tap the electrical output power that can be generated byeach such module, are appropriately established—again, utilizingconventional and well-known techniques for such activity.

It is believed that the disclosure set forth above encompasses multipledistinct inventions with independent utility. While each of theseinventions has been disclosed in its preferred form, the specificembodiments thereof as disclosed and illustrated herein are not to beconsidered in a limiting sense as numerous variations are possible. Thesubject matter of the inventions includes all novel and non-obviouscombinations and subcombinations of the various elements, features,functions and/or properties disclosed herein. No single feature,function, element or property of the disclosed embodiments is essentialto all of the disclosed inventions. Similarly, where the claims recite“a” or “a first” element or the equivalent thereof, such claims shouldbe understood to include incorporation of one or more such elements,neither requiring nor excluding two or more such elements.

It is believed that the following claims particularly point out certaincombinations and subcombinations that are directed to one of thedisclosed inventions and are novel and non-obvious. Inventions embodiedin other combinations and subcombinations of features, functions,elements and/or properties may be claimed through amendment of thepresent claims or presentation of new claims in this or a relatedapplication. Such amended or new claims, whether they are directed to adifferent invention or directed to the same invention, whetherdifferent, broader, narrower or equal in scope to the original claims,are also regarded as included within the subject matter of theinventions of the present disclosure.

What is claimed is:
 1. A photovoltaic structure comprising a polymersubstrate having a support face, and a photovoltaic layer structureformed on and extending over at least a portion of the support face, thephotovoltaic layer structure being responsive to incident light toproduce electrical power, wherein the photovoltaic layer structureincludes a back contact layer and the back contact layer is in a stateof internal compression when the photovoltaic structure is at roomtemperature.
 2. The photovoltaic structure of claim 1, wherein the backcontact layer is sufficiently internally compressed at room temperatureso that it remains in compression at least substantially up to a maximumapplication temperature of the remainder of the photovoltaic layerstructure.
 3. The photovoltaic structure of claim 2, wherein the maximumapplication temperature is at least 350° C.
 4. The photovoltaicstructure of claim 1, wherein the back contact layer includesmolybdenum.
 5. The photovoltaic structure of claim 4, wherein themolybdenum in the back contact layer includes an entrained impurityconstituent.
 6. The photovoltaic structure of claim 4 further includinga metallic interlayer disposed between the back contact layer and thepolymer substrate.
 7. The photovoltaic structure of claim 6, wherein themetallic interlayer contains nickel.
 8. The photovoltaic structure ofclaim 6, wherein the metallic interlayer comprises intermediatecoefficient of thermal expansion (CTE) metals chosen from the groupconsisting of: copper and copper alloys, wherein the alloys comprise atleast one of brass, niobium, chromium, tantalum, and titanium.
 9. Thephotovoltaic structure of claim 1, wherein the substrate includes a backface opposite the support face and a stress-neutralization layer in astate of internal compression disposed over at least part of the backface.
 10. The photovoltaic structure of claim 9, wherein the stressneutralization layer includes molybdenum.
 11. The photovoltaic structureof claim 1, wherein the substrate is a polyimide sheet.
 12. Thephotovoltaic structure of claim 1, wherein the polymer substrate has athickness of less than 0.002 inches.
 13. The photovoltaic structure ofclaim 1, wherein the photovoltaic layer structure includes an absorberlayer chosen from the group consisting of copper-indium-diselenide andcopper-indium gallium diselenide.
 14. The photovoltaic structure ofclaim 1, wherein the photovoltaic structure includes a cadmium-sulfidebuffer layer.
 15. The photovoltaic structure of claim 1, wherein thesubstrate is flexible and the back contact layer is sufficientlyinternally compressed to cause the substrate to curl into a diameter ofless than one inch at room temperature absent a counteracting force. 16.The photovoltaic structure of claim 1, wherein the substrate has acoefficient of thermal expansion at least twice as large as that of theback contact layer.
 17. A flexible, thin-film, photovoltaic structurecomprising a thin, flexible substrate having first and second opposedfaces, a photovoltaic layer structure formed on and extending over atleast a portion of a first face responsive to incident light to producean electrical voltage, and stress-compensating layer structure formed onthe second face acting in conjunction with the photovoltaic layerstructure to maintain the photovoltaic structure in a substantiallyflex-neutral state over a range of temperatures.
 18. The photovoltaicstructure of claim 17, wherein the photovoltaic-layer structure includesa back contact layer and the coefficients of thermal expansion of theback contact layer and the stress-compensating layer are approximatelyequal.
 19. The photovoltaic structure of claim 18, wherein the backcontact layer and the stress-compensating layer structure includemolybdenum.
 20. The photovoltaic structure of claim 19, wherein themolybdenum in the back contact layer and the stress-compensating layerstructure includes an entrained impurity.
 21. The photovoltaic structureof claim 20, wherein the impurity includes oxygen.
 22. The photovoltaicstructure of claim 17, wherein the flexible substrate is a polymer. 23.The photovoltaic structure of claim 17, wherein the flexible substratehas a thickness less than about 0.002 inches.
 24. The photovoltaicstructure of claim 17, wherein the stress-compensating layer structureis in a state of internal compression when the photovoltaic structure isat room temperature.
 25. The photovoltaic structure of claim 17, whereinthe photovoltaic layer structure includes an absorber layer chosen fromthe group consisting of copper-indium-diselenide andcopper-indium-gallium-diselenide.
 26. The photovoltaic structure ofclaim 17, wherein the photovoltaic structure includes a cadmium-sulfidebuffer layer.
 27. The photovoltaic structure of claim 17, wherein thephotovoltaic layer structure includes a conductive top contact layerincluding a wide band-gap semiconductor.
 28. The photovoltaic structureof claim 27, wherein the top contact layer wide band-gap semiconductoris chosen from the group consisting of indium-tin-oxide, aluminum dopedzinc-oxide and boron doped zinc-oxide.
 29. The photovoltaic structure ofclaim 17, wherein the stress-compensating layer structure maintains thephotovoltaic structure in a substantially flex-neutral state from roomtemperature to about 350° C.
 30. The photovoltaic structure of claim 17,wherein the photovoltaic layer structure includes plural,vapor-phase-created sublayers.
 31. A flexible, thin-film, photovoltaic(PV) structure comprising a thin, flexible substrate with first andsecond opposed faces, a PV-cell layer structure disposed over at leastpart of the first face and including an internally compressed backcontact layer, an absorber layer formed of a material selected from thegroup consisting of copper-indium diselenide andcopper-indium-gallium-diselenide, and a window layer componentcomprising sputtered cadmium-sulfide (CdS); and a stress-neutralizationlayer disposed over at least part of the second face to make thephotovoltaic structure substantially flex-neutral at room temperature.32. The photovoltaic structure of claim 31, wherein, absent thestress-neutralization layer, the thin flexible substrate would tend tocurl to a diameter of less than one inch.
 33. The photovoltaic structureof claim 31, wherein the back contact layer comprises molybdenum. 34.The photovoltaic structure of claim 33, wherein thestress-neutralization layer includes molybdenum.
 35. The photovoltaicstructure of claim 34 further including a metallic interlayer betweenthe back contact layer and the flexible substrate.
 36. The photovoltaicstructure of claim 35, wherein the metallic interlayer is a Ni-V alloy.37. The photovoltaic structure of claim 31 further comprising anelectrically conductive top contact layer generally overlying thecadmium-sulfide layer.
 38. The photovoltaic structure of claim 37,wherein the top contact layer includes regions in direct contact withthe cadmium-sulfide layer, and other regions separated from thecadmium-sulfide layer by an electrically insulating sublayer.
 39. Thephotovoltaic structure of claim 38, wherein the electrically insulatingsublayer comprises intrinsic zinc-oxide.
 40. A photovoltaic structurecomprising: a polymer substrate having a support face, and aphotovoltaic layer structure formed on and extending over at least aportion of the support face, the photovoltaic layer structure beingresponsive to incident light to produce electrical power, wherein thephotovoltaic layer structure includes a back contact layer comprising anentrained impurity constituent and the back contact layer is in a stateof internal compression when the photovoltaic structure is at roomtemperature.
 41. The photovoltaic structure of claim 40, wherein theback contact layer comprises molybdenum.
 42. The photovoltaic structureof claim 40, wherein the entrained impurity constituent comprisesoxygen.
 43. The photovoltaic structure of claim 42, wherein a source ofthe oxygen comprises water (H₂O).